The present invention relates to a key telephone system and an interface unit for a key telephone.
In recent years, a key telephone system is widely used in an office or the like. In the key telephone system, a main unit containing a plurality of key telephones is connected to an office line or a public switched telephone network. Data including speech data and control data are transmitted between the key telephone and the main unit in a predetermined data transmission scheme which depends on the system. The data transmission scheme controls the transmission speed and the transmission data format.
The key telephone and the main unit can be independently purchased and therefore respectively updated. However, if the main unit is only updated to be a new one capable of transmitting data at high speed, the conventional key telephones cannot be used since they are not capable of transmitting data at high speed. Therefore, the resources cannot be used efficiently.